Multisensory transfer effects in implicit and explicit category learning

In category learning, transfer emerges when individuals apply the knowledge or strategy learned with one set of stimuli to a novel set. Under multisensory circumstances, it remains unclear whether the transfer occurs differently in implicit and explicit category learning, as it remains controversial whether explicit knowledge is indispensable for transfer. To address this issue, we adopted multisensory stimuli in implicit (i.e., information integration, II) and explicit (i.e., conjunctive rule-based, RB) category learning tasks. Experiment 1 showed that when training and novel sets shared the same feature modalities, spontaneous transfer without feedback was perfect in the II condition but impaired in the RB condition. However, with feedback, the dependent transfer occurred equally between conditions. Experiment 2 revealed that when across different feature modalities, no spontaneous transfer was observed in any of the two conditions. A dependent transfer occurred in the RB but not in the II condition. Experiment 3 found that delayed feedback disrupted II task performance but not RB task performance, indicating that people did not learn the II task using an explicit rule. The current study demonstrated that transfer to novel stimuli could emerge in explicit and implicit category learning within feature modalities. However, the transfer across feature modalities could only arise with feedback in explicit learning. We further discuss these findings' meaning with the current category learning theory.